When a laser beam is shone directly on the lung surface, a glow appears on the pleural surface, surrounding the point of entry. The intensity and pattern of this glow are reproducible, easily measured, and change dramatically with lung volume. Consideration of the physics of reflection, refraction, and absorption of light by the alveolar septa indicates that the pattern of backscattered light may be quantitatively interpreted in terms of the internal dimensions of the alveolar air spaces and the configuration of the alveolar septa. The proposed light scattering technique, then, will give information similar to that obtained by morphometry on fixed specimens. It has the advantage over morphometry that it affords this information readily, continuously, and non-destructively, and can be used under dynamic circumstances during various experimental interventions in live lungs. By contrast, the current method of morphometry is an extremely time consuming process, it destructively evaluates a given lung at only a single point in time, and the geometry itself may be some extent have been altered by the process of fixation. We propose, then, to (1) further develop the theory and technology, (2) to perform the crucial validation of the technique against microscopic morphometry, and (3) to explore the usefulness of the method in a series of physiologic experiments. These studies will include the dependence on lung volume and history, and experiments which have bearing on the nature of surface-tissue interactions, the role of contractile elements in the parenchyma, the role of alveolar collapse and recruitment, the physical state of the lung lining layer under dynamic circumstances, the effects of edema, and the anatomic changes in various lung diseases.